Polysaccharide based aqueous oil well fracturing fluids are typically comprised of two principle components and various ancillary additives. The two principle components are the viscosifying polysaccharide, typically a hydrating gum such as guar gum, hydroxypropylguar or carboxymethylhydroxypropylguar (hereafter collectively referred to as “a guar or derivatized guar” compound) and a crosslinking agent that imparts visco-elastic properties to the viscous fluid. Typically the crosslinking agent will be one of a zirconium, a titanium or a borate compound.
Addition of water soluble borates at a suitable pH to a guar or derivatized guar based fluid usually results in rapid (<20 seconds) crosslinking of the fluid. At times it is desirable to delay the crosslinking of the fluid for a period of time greater than 20 seconds to allow, for example, the fluid to enter an oil and/or gas bearing formation as a viscous fluid and then have the crosslinking occur whilst in the formation.
To that end, in U.S. Pat. No. 4,619,776 of Mondshine describe delayed borate crosslinking agents using calcium and borate containing minerals such as colemanite, ulexite and probertite. These fluids are also advantageous for stability. As the temperature of the fracturing fluid increases, the solubility of these borate minerals increases releasing further borate ions which stabilize the system. The ability to regulate, to some degree, the rate at which products crosslink by varying the ratios of colemanite, calcined colemanite and ulexite has been disclosed. Colemanite by itself is not useful since the delay times are frequently too great.
Harris, Jr. in U.S. Pat. No. 6,743,756 has described the use of dehydrated boric acid and boric acid salts for use as delayed crosslinkers. High temperature stability may be problematic in these systems. While the materials disclosed in this reference can be added at concentrations that are sufficient to impart higher temperature stability, at these higher concentrations the desired delay in crosslinking no longer occurs. In particular, the crosslink is substantially instantaneous.